Calculate pH, pOH, and hydrogen ion concentrations in aqueous solutions. Free online chemistry pH calculator with step-by-step results.
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Enter a pH value between 0 and 14
Enter a pOH value between 0 and 14
pH
7.00
−log₁₀[H⁺]
pOH
7.00
−log₁₀[OH⁻]
[H⁺]
1.00 × 10⁻⁷ M
Hydrogen ion concentration
[OH⁻]
1.00 × 10⁻⁷ M
Hydroxide ion concentration
Solution Type
Neutral
At 25°C
📝 Step-by-Step Calculation
Real-World pH Examples
🧪 Pure Water at 25°C
Pure water has an [H⁺] concentration of 1.0 × 10⁻⁷ M.
pH: −log₁₀(1.0 × 10⁻⁷) = 7.00
pOH: 14.00 − 7.00 = 7.00
[OH⁻]: 1.0 × 10⁻⁷ M
Pure water is neutral — the concentrations of H⁺ and OH⁻ are equal.
🍋 Lemon Juice (Acidic)
Lemon juice has a hydrogen ion concentration of approximately 3.16 × 10⁻³ M.
pH: −log₁₀(3.16 × 10⁻³) = 2.50
pOH: 14.00 − 2.50 = 11.50
[OH⁻]: 10⁻¹¹·⁵⁰ = 3.16 × 10⁻¹² M
Strongly acidic with high H⁺ concentration and very low OH⁻.
🧴 Household Ammonia (Basic)
Household ammonia has a pOH of 3.00.
[OH⁻]: 10⁻³·⁰⁰ = 1.0 × 10⁻³ M
pH: 14.00 − 3.00 = 11.00
[H⁺]: 10⁻¹¹·⁰⁰ = 1.0 × 10⁻¹¹ M
Strongly basic with high OH⁻ concentration and very low H⁺.
🩸 Human Blood
Human blood typically has a pH of 7.40.
[H⁺]: 10⁻⁷·⁴⁰ = 3.98 × 10⁻⁸ M
pOH: 14.00 − 7.40 = 6.60
[OH⁻]: 10⁻⁶·⁶⁰ = 2.51 × 10⁻⁷ M
Blood is slightly basic (alkaline). A pH below 7.35 (acidosis) or above 7.45 (alkalosis) requires medical attention.
Understanding pH and pOH
The pH scale measures how acidic or basic (alkaline) a solution is. It ranges from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral at 25°C. The scale is logarithmic, meaning each whole number change represents a tenfold change in hydrogen ion concentration.
Core Formulas
pH = −log₁₀[H⁺]
pH is the negative base-10 logarithm of the hydrogen ion concentration in mol/L.
pOH = −log₁₀[OH⁻]
pOH is the negative base-10 logarithm of the hydroxide ion concentration.
pH + pOH = 14.00 (at 25°C)
The sum of pH and pOH is always 14 at 25°C (298 K). This changes with temperature.
[H⁺] = 10⁻ᵖᴴ [OH⁻] = 10⁻ᵖᴼᴴ
To convert from pH/pOH back to concentration, raise 10 to the negative power.
The pH Scale
0–3 Strong Acid
4–5 Weak Acid
6 Slightly Acidic
7 Neutral
8 Slightly Basic
9–10 Weak Base
11–14 Strong Base
How to Calculate pH Step by Step
1
Identify the given: Determine whether you know [H⁺], pH, or pOH
2
Apply the formula: If you have [H⁺], use pH = −log₁₀[H⁺]. If you have pH, use [H⁺] = 10⁻ᵖᴴ
3
Calculate pOH: Use pOH = 14 − pH (at 25°C)
4
Find remaining values: [OH⁻] = 10⁻ᵖᴼᴴ, or from [H⁺] using K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴
Use scientific notation like 1e-7 or 3.16e-3. The calculator accepts both decimal and exponential formats for convenience.
🌡️ Temperature Matters
At 25°C, pH + pOH = 14. At other temperatures, the ion product of water (Kw) changes, and the neutral pH shifts away from 7.
🚫 No Zero Concentration
You cannot take the logarithm of zero. If your H⁺ concentration is zero (pure water with no ions), use 1.0 × 10⁻⁷ M as the minimum.
🔢 Significant Figures
Only the digits after the decimal point in a pH value are significant. A pH of 7.00 has two significant figures, corresponding to two digits in the concentration.
🧪
pH from [H⁺]
Enter any hydrogen ion concentration and instantly compute the pH and pOH values with full step-by-step explanation.
🔬
Concentration from pH
Convert pH to hydrogen ion concentration using the inverse logarithm formula. See [H⁺] in scientific notation.
⚗️
OH⁻ & pOH Support
Calculate hydroxide ion concentration from pOH, or find pOH from pH using the autoionization constant of water.
📝
Step-by-Step Solutions
Every calculation includes a detailed breakdown — from logarithms to concentration conversions — making learning easy.
pH (potential of hydrogen) is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺]. The scale runs from 0 to 14, with 7 being neutral at standard temperature (25°C).
The pH scale is logarithmic, not linear. This means a solution with pH 3 is ten times more acidic than a solution with pH 4, and one hundred times more acidic than a solution with pH 5. This logarithmic relationship allows the scale to conveniently represent the enormous range of hydrogen ion concentrations found in nature — from approximately 10 M (pH ≈ −1) in concentrated acids to 10⁻¹⁴ M (pH ≈ 15) in strongly basic solutions.
The concept of pH was introduced by Danish chemist Søren Peder Lauritz Sørensen in 1909 at the Carlsberg Laboratory. Originally written as "pH" standing for "power of hydrogen" (German: Potenz der Wasserstoffionen), it has become one of the most fundamental and widely used measurements in chemistry, biology, medicine, agriculture, and environmental science.
Understanding Acidity and Basicity
Acidic solutions (pH < 7): High [H⁺] concentration, low [OH⁻]. Examples include lemon juice (pH ≈ 2), vinegar (pH ≈ 3), and stomach acid (pH ≈ 1.5–3.5)
Neutral solutions (pH = 7): Equal [H⁺] and [OH⁻] concentrations (1.0 × 10⁻⁷ M each). Pure water is the classic example
Basic (alkaline) solutions (pH > 7): Low [H⁺] concentration, high [OH⁻]. Examples include baking soda (pH ≈ 9), household ammonia (pH ≈ 11), and drain cleaner (pH ≈ 14)
Why pH Matters
pH is a critical parameter in countless scientific, industrial, and everyday contexts. Here's why understanding and measuring pH is so important:
🧬 Biology & Medicine
Human blood must maintain a pH between 7.35 and 7.45. Even small deviations can be life-threatening. Enzymes function optimally at specific pH ranges — stomach pepsin works at pH 2, while intestinal enzymes need pH 8.
🌱 Agriculture
Soil pH affects nutrient availability for plants. Most crops grow best in slightly acidic to neutral soil (pH 6.0–7.0). Blueberries prefer acidic soil (pH 4.5–5.5), while asparagus thrives in more alkaline conditions.
🏭 Industry
pH control is vital in water treatment, food production, pharmaceuticals, and manufacturing. Wastewater must be neutralized before discharge. The optimal pH for swimming pools is 7.2–7.8.
🌍 Environmental Science
Ocean acidification — the decrease in ocean pH due to CO₂ absorption — threatens marine ecosystems. Acid rain (pH 4.0–5.0) damages forests, lakes, and buildings. Monitoring pH helps track environmental health.
Frequently Asked Questions
What is the difference between pH and pOH?
pH measures the hydrogen ion concentration [H⁺] in a solution, while pOH measures the hydroxide ion concentration [OH⁻]. They are inversely related through the equation pH + pOH = 14 (at 25°C). A low pH means high acidity and low pOH; a high pH means low acidity and high pOH. In neutral solutions, both pH and pOH equal 7.
Why is the pH scale from 0 to 14?
The pH scale ranges from 0 to 14 because the ion product of water (Kw = [H⁺][OH⁻]) equals 1.0 × 10⁻¹⁴ at 25°C. The most acidic solution possible has [H⁺] ≈ 1 M (pH = 0), and the most basic has [OH⁻] ≈ 1 M (pH = 14). However, pH values below 0 and above 14 are possible for extremely concentrated acids or bases — for example, 10 M HCl has a pH of approximately −1.
How do I convert between pH and hydrogen ion concentration?
To convert from [H⁺] to pH, use the formula pH = −log₁₀[H⁺]. For example, if [H⁺] = 1.0 × 10⁻³ M, then pH = −log₁₀(1.0 × 10⁻³) = 3.00. To convert from pH to [H⁺], use [H⁺] = 10⁻ᵖᴴ. For example, if pH = 5.50, then [H⁺] = 10⁻⁵·⁵⁰ = 3.16 × 10⁻⁶ M. Our calculator handles both conversions automatically.
Does temperature affect pH?
Yes, temperature significantly affects pH values. The ion product of water (Kw) changes with temperature — at 0°C, Kw = 1.14 × 10⁻¹⁵ (pH neutral = 7.47), while at 60°C, Kw = 9.55 × 10⁻¹⁴ (pH neutral = 6.51). This means the pH of pure water decreases as temperature increases, even though the water remains neutral. The relationship pH + pOH = pKw always holds, but pKw changes with temperature. Our calculator assumes standard conditions (25°C) where pKw = 14.00.
What is the relationship between [H⁺] and [OH⁻]?
In any aqueous solution at 25°C, the product of [H⁺] and [OH⁻] is always constant: [H⁺] × [OH⁻] = Kw = 1.0 × 10⁻¹⁴. This is called the autoionization constant of water. This means if you know either [H⁺] or [OH⁻], you can calculate the other: [OH⁻] = 1.0 × 10⁻¹⁴ ÷ [H⁺]. For example, if [H⁺] = 1.0 × 10⁻⁴ M, then [OH⁻] = 1.0 × 10⁻¹⁴ ÷ 1.0 × 10⁻⁴ = 1.0 × 10⁻¹⁰ M.
What does a negative pH value mean?
A negative pH occurs when the hydrogen ion concentration exceeds 1 M (10⁰ M). For example, 12 M HCl (concentrated hydrochloric acid) has [H⁺] = 12 M, giving pH = −log₁₀(12) = −1.08. Negative pH values are uncommon in everyday life but occur in highly concentrated strong acids. Similarly, pH values above 14 are possible for highly concentrated strong bases. Our calculator supports pH values outside the 0–14 range for these special cases.
⚠️ Important Note: This pH Calculator is for educational and informational purposes only. While every effort has been made to ensure accuracy, results should be verified independently for critical applications such as laboratory work, industrial processes, medical diagnostics, or environmental assessments. Always use calibrated pH meters and appropriate analytical methods for precise measurements in professional contexts.